The Nervous System: Sensory Receptors, Neurons, and Impulses
Nervous System: Functions and Components
The nervous system governs the life of relation. It processes information received to develop appropriate responses.
Sensory Receptors
Stimuli are all internal and external changes capable of generating a response from the living being. Receptors are specialized cells that capture and translate stimuli into nerve impulses. These nerve cells are connected chemically or electrically with two neurons.
Classification of Receptors
By Location
- Interoceptors: Located in the walls of the visceral organs.
- Proprioceptors: Located in the muscles, tendons, and joints.
- Exteroceptors: Perceive stimuli from the external environment and are located in the sense organs.
By Stimuli Type
- Photoreceptors: Respond to light stimuli (located in the eyes).
- Chemoreceptors: Respond to chemical stimuli (located in the mouth and nasal cavity).
- Mechanoreceptors: Respond to mechanical stimuli (located in the skin).
- Nociceptors: Respond to pain stimuli (located in the skin, muscles, and joints).
Nerve Impulse
A nerve impulse is the transmission of electrical signals from one neuron to another or from one neuron to another cell.
Effector Organs
Effector organs carry out the responses prepared by the central nervous system. These include muscles and glands; the former contract and the latter secrete substances upon receiving an order.
Stimulus → Receptor → Central Nervous System → Effector Organs → Response
Brain
The brain is involved in the regulation of posture and body balance. It is the control center for intellectual and voluntary movements.
Spinal Cord
The spinal cord carries information from the brain to the body and from the body to the brain. It also controls basic reflexes.
Neuron
Neurons are the cells of nervous tissue; their function is the conduction of nerve impulses.
Classification of Neurons (According to Their Functions)
- Association neurons or interneurons: Receive, process, and send messages to every corner of the body. They make up 99% of neurons and are located in the CNS.
- Motor neurons: Carry impulses from the CNS to muscles or glands. They are located in the CNS, and their axons are part of a peripheral nerve. They have an efferent direction.
- Sensory neurons: Carry impulses from all receptors sequentially from the body (located in the sense organs) to the CNS (brain and spinal cord). They have an afferent direction.
Neuroglia
Neuroglia are a set of cells that do not conduct impulses but are responsible for the protection and nourishment of neurons.
The neuron contains cytoplasmic organelles, such as Nissl bodies or tigroid bodies (sets of ribosomes responsible for the synthesis of neuronal proteins). These substances are enveloped by membranes provided by the Golgi apparatus, forming secretory vesicles. These travel along the dendrites, traversing the axon, to be located in the terminal buttons, forming presynaptic vesicles.
Neuron States
- Neuron at rest (polarized): In this state, there is no transmission of a nerve impulse. The intracellular environment has negative charges, and the extracellular environment has positive charges.
- Neuron depolarized (transmits nerve impulses): The charges are reversed; this change generates nerve impulses that gradually spread along the entire neuron.
- Neuron returning to its initial state (repolarized): The neuron gradually returns to its initial state when the nerve impulse reaches the telodendrion.
For this process to take place, the primary action needed is a stimulus that must reach the “threshold of excitation.”